You probably multiplied the numbers and the powers of 10 correctly, but when you converted 42 to $\displaystyle{4.2}\times{{10}}^{{1}}\usepackage{cmbright}\Huge\$ you forgot to regroup the powers of 10 and find $\displaystyle{4.2}\times{\left({{10}}^{{1}}\times{{10}}^{{-{{4}}}}\right)}\usepackage{cmbright}\Huge\$ .